No native completion with python3 in Emacs 25.2

Question

On Emacs 25.2 and Python 3.5 on an Centos-like Oracle Linux I have the issue that invoking a python shell gives me no completion. To reproduce, start

$ emacs -Q --eval '(setq python-shell-interpreter "python3")'

then execute

M-x run-python

This produces the warning

Warning (python): Your ‘python-shell-interpreter’ doesn’t seem to support readline, yet ‘python-shell-completion-native’ was t and "python3" is not part of the ‘python-shell-completion-native-disabled-interpreters’ list.  Native completions have been disabled locally.

It is the same problem as Your ‘python-shell-interpreter’ doesn’t seem to support readline, but:

1) I am already on Emacs version 25.2, so the fixfrom one answer does not help (because 25.2 contains the proposed fix).

2) I am on bash 4.1 and have no ~/.inputrc file, so the fix from the other answer doesn't help either.

How can I fix this?

Update:

On my MacOS machine the initialisation works correctly. However on MacOS I use Emacs 25.3. Therefore I installed 25.3 (from source ) on the Oracle Linux system, too - the Python completion still fails, because of same above mentioned issue!

Answer 1

The native completion mechanism depends on the gnureadline python module. You can check if it's present by executing

import readline
print(readline.__doc__)

at a python shell. The message

ImportError: No module named readline

indicates it's missing. It can be installed with pip install gnureadline. When it's correctly installed, the above code should print:

Importing this module enables command line editing using GNU readline.

The readline module might be missing on Oracle Linux for Python versions which are installed from source.